The invention relates to a semi-sequential transmission system for transmitting, by short wave link, signals between data acquisition apparatus of a seismic reception assembly and a central control and recording laboratory by means of several transmission frequencies and with automatic distribution of the different available frequencies between the acquisition apparatus.
1. Field of the Invention
Modern seismic prospection methods comprise the use of data acquisition apparatus spread out at even intervals over a distance sometimes of several kilometers. They are each adapted for collecting seismic signals picked up by one or more appropriate receivers (hydrophones or geophones) in response to shocks transmitted into the ground by a seismic source and reflected by the discontinuities of the subsoil. The signals which they collect are sampled, digitized and stored in a memory before being transmitted in real or deferred time to a central control and recording laboratory.
2. Description of the Prior Art
Different radio transmission systems making it possible to centralize seismic data collected by acquisition apparatus are described in the French patent application FR No. 2 599 533, in the French patents FR Nos. 2 511 772 and 2 197 182, in the British patent GB No. 2 055 467 or the U.S. Pat. Nos. 4,152,691, 4,086,504, 3,987,406, for example.
Transmission of the data collected by the acquisition devices may also take place sequentially, each of them transmitting its own data in turn either directly to the central laboratory or through other intermediate acquisition devices or relay elements. Recording means are used for storing the data collected, during the time necessary for its sequential transfer to the central laboratory.
These methods only require a wide band short wave channel. But if the number of acquisition apparatus successively interrogated is large, the total transfer time and so the time interval between two successive seismic transmission-reception cycles become great, which contributes to lengthening the time for exploring the seismic profile studied.
Transmission to the central laboratory of the collected data may take place in real time and simultaneously for all the acquisition apparatus. That requires the use and so the availability of a large number of wide band or narrow band short wave transmission channels, of different frequencies. The use of such a system is often difficult in practice. Each of the apparatus must be equipped with a radio transmitter-receiver individualized by a particular transmission frequency and the central laboratory with selective detection assemblies for separating the signals modulating the different carriers. The preparatory adjustment and setting up work is made longer and more complex. Furthermore, the simultaneous transmission of data is often disturbed by cross modulation phenomena occurring on reception at the central laboratory between carriers of very different levels which emanate from near-by transmitters and more distant transmitters.
The use of a combined transmission system in which a limited number of acquisition apparatus simultaneously transfer to the central laboratory the data which they have collected by means of a certain number of different carrier frequencies may form an interesting alternative to the extent that the total time for centralizing the data may thus be reduced. But the setting up of such a system is delicate. The different acquisition apparatus after installation in the field must be individualized. Each of them is then adjusted so as to transmit or receive on one frequency from the available frequencies. Since the acquisition apparatus are not necessarily disposed in the field by respecting the increasing order of their respective serial numbers, for practical positioning reasons, the operator who adjusts them must record the number of the radio channel associated with each of them. This may be a source of error, particularly when a defective acquisition apparatus must be replaced by another whose serial number is different.
The transmission method and system of the invention avoids the above drawbacks.